The present invention relates to a semiconductor device.
Silicon dioxide (SiO2) has excellent insulation properties having a band gap as large as 8 to 9 eV and is often used as material for gate insulating layers, inter-layer insulating layers or the like in a semiconductor device.
However, recently, gate insulating layers are required to be thinned due to miniaturization of semiconductor devices and oxide layers having the thickness of 3.0 nm or less are used. When the thickness of the oxide layers is thinned to 3.0 nm or less, there is a problem that a tunnel current is increased as it cannot be disregarded, so that a leakage current is increased to thereby increase the power consumption.
It is known that the tunnel current is mainly divided into a Fowler-Nordheim tunnel current (FN current) and a direct tunnel current (DT current). The FN current is the current flowing by causing electrons to tunnel the triangle potential generated by bending the energy barrier by an external electric field. The DT current is the current flowing by causing electrons to tunnel an insulating layer directly. The current having a problem in the thinned silicon dioxide (SiO2) is the DT current.
It is considered that material named high-dielectric constant (high-k) material having the dielectric constant higher than that of the silicon dioxide (SiO2) such as, for example, zirconium dioxide (ZrO2), hafnium dioxide (HfO2), titanium dioxide (TiO2) or the like having the relative dielectric constant of about 25 is used for the gate insulating layer to thereby thicken the thickness of the gate insulating layer and suppress increase of the leakage current while maintaining the dielectric properties. The relative dielectric constant of the silicon dioxide is about 3.9 and accordingly the thickness of the high-dielectric constant insulating layer, that is, the high-k insulating layer having the same dielectric properties as the silicon dioxide having the thickness of 2 nm, for example, is about 12.8 nm when the relative dielectric constant of the high-k insulating layer is equal to 25. The high-k thin film layer having the thickness of 12.8 nm is equivalent to the silicon oxide having the thickness of 2 nm that is named equivalent oxide thickness. The actual thickness of 12.8 nm is named the physical thickness.
In this connection, it is confirmed that a leakage current induced by the mechanical stress in the semiconductor device. Japanese Patent Publication JP-A-2002-246591 discloses that the leakage current is increased depending on the manufacturing method of gate electrodes or gate insulating layers when the high-k material is used for the gate insulating layer.